The present invention relates to a semiconductor device and, more particularly, to an improvement in a passivation structure of a high breakdown voltage element such as a power transistor.
Guard-ring structure and resistive field structure provide a planar transistor with a high breakdown voltage of, for example, 1,000 V or more. In a guard-ring structure, a ring-like diffusion layer is formed around a base diffusion layer. In a resistive field plate structure, a high resistance film is formed between a base electrode and an equi-potential ring (EQPR) collector electrode. A potential difference is applied across both ends of the high resistance film to create a potential gradient from the base region to the equipotential ring collector region.
Takeshi Matsushita et al. described in "Highly Reliable High Voltage Transistors By Use of the SIPOS Process," "IEEE Transactions on Electron Devices," Vol. ED. 23, No. 8, August 1976, a semi-insulative polysilicon (SIPOS) film as a passivation structure of a power transistor. The passivation structure prevents a drop in the breakdown voltage and a decrease in reliability due to an external charge. In the passivation structure, the SIPOS film is formed by direct deposition of semi-insulative polysilicon doped with oxygen onto a substrate surface using a CVD method.
A SIPOS film has the following electrical characteristics:
(1) Shield effect
The SIPOS film is electrically almost neutral, and shows semi-insulating characteristics. As a result, when a charge is applied to a structure comprising a silicon substrate--a SIPOS film--an insulation film, the charge is shielded by the SIPOS film and does not enter the silicon substrate. Therefore, influence on the silicon substrate from external charges is prevented.
(2) Hot carrier release effect
In a SIPOS film, carriers move by hopping between states which are continuous in the energy band gap. This means that when a reverse bias is applied to the pn junction in the substrate, the hot carriers injected in the SIPOS film are released.
(3) Switching effect
The conductivity of a SIPOS film varies with the intensity of the electric field in the film. If the field intensity exceeds 2 or 3.times.10.sup.5 V/cm, the conductivity will drop in the order of 100 to 1,000,000 times.
Conventionally, a combination of a guard-ring structure and a SIPOS film is known as a means of obtaining a power transistor with a high breakdown voltage and good reliability. FIG. 1 is a sectional view of a portion of such a structure. In FIG. 1, reference numeral 1 denotes an n.sup.- -type semiconductor substrate; 2, a collector region comprising an n.sup.+ -type diffusion layer formed on the lower surface of substrate 1; 3, a base region comprising a p.sup.+ -type diffusion layer formed on a portion of the upper surface of substrate 1; 4, an equi-potential ring (EQPR) collector region comprising an n.sup.+ -type diffusion layer formed on a portion of the upper surface of substrate 1; and 5, a guard ring region comprising a p.sup.+ -type diffusion layer formed on the portion of the upper surface of substrate 1 to surround base region 3. Three guard-ring regions are usually provided to surround triple base region 3. However, for the sake of clarity, only one region is shown. Reference numeral 6 denotes a SIPOS film deposited on the upper surface of substrate 1; 7, an insulation (SiO.sub.2) film formed on film 6 and extending onto base region 3 and EQPR collector region 4; 8, an electrode (normally aluminum) formed on region 3; 9, an electrode (normally aluminum) formed on region 4; and 10, a passivation formed film on electrodes 8 and 9 and film 7.
In order to increase the reliability of a transistor with the resistive field plate, a high resistance SIPOS film is conventionally proposed as the field plate 11, as shown in FIG. 2. In FIG. 2, insulation (SiO.sub.2) film 7 is formed on the upper surface of substrate 1, and SIPOS film 11 is formed on film 7. Film 11 connects electrodes 8 and 9. The same reference numerals in FIGS. 1 and 2 denote the same parts. The transistor with this structure, compared with the above-described transistor having triple guard-rings, has a smaller chip area, and is free from an abnormal impurity diffusion from the guard-rings and pinholes between the guard-rings.
In the transistor having the guard-ring structure and SIPOS film shown in FIG. 1, in order to resolve the two problems of shielding against external charges and release of hot carriers injected from the substrate, it is necessary to increase the thickness of film 6 to 1.0 .mu.m or more so that the concentration of doped oxygen is reduced. However, because of variations in impurity diffusion into the guard ring region and in the composition of film 6 which is sensitive to CVD processing conditions, conductivity will drop greatly in film 6, and the leakage current near breakdown voltage level increases to cause the reverse-bias waveform to go soft.
FIG. 3 shows the relationship of collector-base voltage V.sub.CB to collector-base reverse current I.sub.CB. The switching characteristics in film 6 affect breakdown characteristics, which develop a soft waveform as shown in FIG. 4. Transistors having breakdown characteristics shown in FIG. 4 thermally break down in bias (B) - temperature (T) stress tests. As a result, it is difficult to improve reliability and manufacturing yield.
In the transistor shown in FIG. 2, which uses a high resistance film (a SIPOS film) as resistive field 11, because of the potential gradient in plate 11, the electric field on the surface of the substrate is weak, and the injection of hot carriers into film 7 is reduced. However, since film 7 is formed on the upper surface of substrate 1, improvements in reliability coming from the injection release of hot carriers by film 11 cannot be satisfactorily obtained.